The invention relates to a semiconductor memory using an electrically rewritable non-volatile memory and, more particularly, to a technique for realizing high speed in rewriting of data and for improving the life of a rewriting operation.
U.S. Pat. No. 4,908,793 discloses a disk storage having a battery and a semiconductor memory.
In the conventional apparatus, as disclosed in U.S. Pat. No. 4,908,793 or JP-A-63-100555, the non-volatile memory (magnetic disk) is nothing but an auxiliary memory to save data in the volatile memory (RAM) when a power source is shut off. In the ordinary operating state, data is written into or read out from the volatile memory. In JP-A-2-81398, an EEPROM is used as a non-volatile memory.
In the above conventional technique, the non-volatile memory for saving data is indispensable in spite of the fact that it is not ordinarily used and the non-volatile memory needs to have a memory capacity which is equal to or larger than that of the volatile memory. Therefore, the memory capacity which can be actually used is equal to or less than 50% of the total memory capacity of both the volatile memory and the non-volatile memory. There are problems with respect to points that efficiency of the non-volatile memory is very low and the overall costs of the memories are also expensive.
Further, in a memory device using only the magnetic disk as a non-volatile memory in order to realize a non-volatile memory device, the time which is required to read out or write data is so long as to be tens of milliseconds. In a memory device using only an EEPROM, the time which is required to read out data is so long as to be a few milliseconds. There is, consequently, a problem from a viewpoint of a high accessing speed of data. On the other hand, as for the EEPROM, up to tens of thousands of rewriting times of data are assured, so that there is also a problem in terms of the life of the memory device.